ELECTRIC RAILWAY ENGINEERING

The motors act as generators if operated slightly above their
synchronous speed, thus permitting regenerative braking on
the down grades. Their starting characteristics are inferior
to those possessed by series type motors and therefore they are
not so well adapted for use in a service where the number of
stops per mile is large. This, and the fact that small inequalities
in drive wheel diameters result in serious overloads on the
motors geared to the wheels with the larger diameters, prac-
tically precludes the use of this type of motor for all classes of
service except that of long haul, locomotive drawn trains.

Rolling Stock.—Two classes of rolling stock, namely, motor
cars and locomotives, will be compared.

The added weight and lower acceleration for a given capacity
of motor are not the only disadvantages of the single-phase
alternating current system from the standpoint of motor car
rolling stock. As has been pointed out, it is often desirable
that cars equipped for alternating current service be able to
enter cities upon direct current. While the alternating current
single-phase series motor makes an excellent direct current
motor, the control equipment for use upon either system is at
best rather complicated and its first cost, weight and maintenance
relatively high. The added complication of this combined control
is at once obvious if a comparison be made of Figs. 143 and 149,
while the tables listed under the caption "First Cost, Mainte-
nance, Operating Expense" prove the rolling stock thus equipped
to cost 28 per cent, more, with a probable maintenance charge
of 49 per cent, more than the 600 volt direct current equipment.

Since the three-phase and split-phase systems make use of
three-phase motors, they are not very well suited for motor car
operation.

Single-phase rectifier motor cars should be a little lighter than
straight single-phase cars, have about the same overall efficiency,
and would possess the excellent accelerating characteristics
of the direct current car. The rectifier set would take up some
valuable floor space, however, and the maintenance charges
would be higher than for the direct current.car.

In the matter of locomotive weights, there seems to be little
difference between the direct current and the three-phase systems.
Single-phase and split-phase locomotives are somewhat heavier
although the differences are not great. Inasmuch as it is
sometimes necessary to ballast electric locomotives in order to